Page:The Osteology of the Reptiles.pdf/271



, aquatic reptiles with long neck, body, and tail, two or three feet in length. Structure of skull imperfectly known, probably with a single, upper temporal opening on each side. Face long and slender, the nostril near orbits, the premaxillae elongated. Teeth numerous, long and slender; small teeth on vomers, probably also on other palatal bones. Vertebrae deeply amphicoelous; intercentra unknown; eleven or twelve cervicals, eighteen to twenty-two dorsals, two sacrals and sixty or more caudals. Free ribs on all presacrals except atlas; dorsal ribs stout, single-headed, articulating with centra. Numerous parasternal ribs. Scapula fan-shaped; a single coracoid; clavicular girdle primitive; pelvis with small pubo-ischiatic vacuity. Humerus with entepicondylar foramen. Propodials long; epipodials short, carpus and tarsus primitive; phalangeal formula of pes (in Mesosaurus and Noteosaurus at least) 2, 3, 4, 5, 6, the fifth toe elongate.

These small reptiles, the first known in geological history with marked aquatic adaptations, retain many primitive characters, though highly specialized in the scapular girdle with its single coracoid, the earliest known. Aside from the Dolichosauria and certain dinosaurs they are the only known aquatic reptiles with both neck and tail elongated. Until the skull is better known, however, doubt remains as to their relationships with other reptiles. By some they have been placed with the double-arched reptiles; by others among the Sauropterygia. Because of the articulation of the single-headed ribs especially, and the probable possession of but a single, upper temporal opening, their natural association seems to be near the ichthyosaurs and lizards.